Fontconfig packaging tips

From FedoraProject

Fontconfig directives will help you control our font substitution rules. While synthetic font compositions are not a complete substitute to good pan-unicode fonts [1], they're the next best thing. You should always consider adding fontconfig tuning to your font packages.

Review required Before packaging a new (custom-created or upstream-created) fontconfig ruleset, please post it on the SIG list for review.

Code snippets only The examples given on this page are code snippets. They won't work if you do not add the usual XML wrapper around them.

Config file considerations

Choosing a ruleset numeral prefix

Fontconfig will sort the ruleset files dropped in /etc/fonts/conf.d/ by filename before evaluating them. Therefore the rules of 01-xxx.conf and 02-yyy.conf will have the same effect a single 01-xy.conf file containing first xxx rules then yyy rules.

Usually that means the files with the smaller prefix will have higher precedence. Unfortunately the fontconfig syntax is pretty flexible and allows prepending effects to existing rules. Careful testing is therefore required if your package deploys rules that interact with the existing fontconfig ruleset.

Font substitution

You as a packager know your font is a good substitute to another one, because it has the same metrics, has been forked from the same root, etc. fontconfig unfortunately has no such knowledge. You can teach fontconfig to consider your font first when an application requests this other font by using the following rules.

Adding this rule to your package will help fontconfig choose the right generic family to substitute glyphs from if the coverage of your font is insufficient for a run of text.

Registering a font in default families

Fontconfig rules can also be used to add fonts in the substitution lists used to define synthetic default fonts like sans-serif. The effects of those rules are highly dependent on the numeral prefixes of the filenames used.

The following rules will change priorities for every language It is strictly forbidden to pre-empt current distribution defaults without consulting the Fedora fontconfig maintainer and the Fonts SIG list first. To avoid pre-empting defaults, use high numbers in the prefix range appropriate for your font.

Simple priority lists

The substitution lists for default families are built using the following blocks:

When matching a family, fontconfig will use the first suitable font in its substitution list. Thus <family> elements within a <prefer> block are ordered by descending priority, and rulesets with low numeral prefixes take precedence over rulesets with high numeral prefixes.

Locale-specific overrides

The main default of simple substitution lists is they order whole fonts without taking the language being rendered into account. This would work fine if fonts never overlapped. All but the simplest real-world fonts, however, cover multiple scripts[2]. And the Unicode consortium itself made the dubious choice of regrouping different regional variants of some glyphs at unique codepoints.

If a font contains a very desirable drawing of script A, and a botched drawing of script B, you can not assign it a high priority for the sake of script A users without making script B users miserable. Since there are only so many good free/libre-open fonts, if the first group is numerous avoiding this font for the sake of the second group may not be possible.

Sometimes the drawing conventions of the same script are not the same in different locales, so putting any font that contains this script in a default font substitution list will make some users miserable[3]. Yet you do need to support users of this script in default fonts too.

Therefore it's very difficult to create a setup that pleases everyone using fontconfig default substitution lists only. Some locales will be sacrificed, which is not what we want. We need to complement the default lists using another method:

This rule will override default substitution lists for a particular locale. Since it uses the prepend mode, it follows an inverted priority: rulesets with high numeral prefixes take precedence over rulesets with low numeral prefixes. If you need to ship such a rule, please use the following prefix range:

Fontconfig config file ordering, continued

Prefix

Description

65-69

Fonts with less common encodings, ending with fonts that provide coverage of exotic unicode blocks at the expense of drawing quality

Auto-scaling problem fonts

Sometimes a font author designed a font around a size that just does not fit in with others. The following rule will autoscale such a font to fix the problem:

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